The present invention relates to vibration detectors, in particular, it relates to a vibration detector which is capable of detecting the condition that two out of three components of a composite vibration have reached a predetermined value.
The present vibration detector can be utilized as a seismograph for measuring an earthquake. Usually, the vibration of an earthquake has three independent components, however, the analysis of two components in an horizontal plane is enough for determining earthquake damage.
There are various known vibration detectors, some of which are as follows.
(1) Vibration Detector (A)
Two sets of conventional vibrometers which detect only one component are appropriately arranged so that two components may be detected. The electric output from these two sets of vibrometers is delivered to the square root circuit and it is then determined whether the output thus produced has reached the set point.
(2) Vibration Detector (B)
A pendulum which is allowed to vibrate within an approximate plane doubles as a movable electrode. This pendulum in the stationary position is positioned in the centre of a cylindrical ring which is the fixed electrode. As the vibration reaches the predetermined value, the movable electrode makes contact with the fixed electrode, and a circuit is closed to provide an electrical signal.
(3) Vibration Detector (C)
In the vibration detector (B) in place of the movable electrode and the fixed electrode, multiple numbers of electric contacts are provided either on the circumference of the pendulum or on that of the cylindrical fulcrum of the pendulum. As the vibration reaches the set point the circuit is closed at the electric contact points.
The following are detailed descriptions with reference to FIGS. 1 to 3 (B) of the accompanying drawings, of each of the above-mentioned three different prior art vibration detectors used in seismography to detect horizontal components of earthquake motion.
FIG. 1 depicts an example of the first form of detector, (detector A). In FIG. 1, 1a is the vibrometer installed to detect the east-west component of the earthquake motion, 1b is the other vibrometer for detecting the south-north component of the earthquake motion, 3a is a square root circuit, 4a is an amplitude detector, 5a is a relay circuit, 6a is a relay output.
This vibration detector detects the movement of the ground in a horizontal plane. The vibrometer or seismograph converter 1a for detecting the east-west component delivers electrical output corresponding to the amplitude of the east-west vibration of the earthquake motion to the square root circuit 3a. The vibrometer seismograph converter 2a for detecting the south-north component delivers electrical output corresponding to the vibration of the south-north component of the earthquake motion to the square root circuit 3a. The square root circuit 3a computes the square root of the above two inputs, and applies an electric output corresponding to the amplitude of the vibration of the earthquake in the horizontal plane to the amplitude detector 4a. The amplitude detector 4a generates output, when an electrical input exceeds a predetermined value. This output actuates the relay circuit 5a, and the relay contact output 6a provides an output signal. If the relay circuit 5a is a self-hold type, the ablve mentioned output signal is maintained after the vibration stops.
However, this type of device requires two sets of seismograph converters 1a and 1b. Furthermore, electronic circuits of various functions are needed. Therefore, this device is complex and high priced.
FIG. 2 shows the second model (vibrator B). In FIG. 2, the reference numeral 1b is a platform, 2b is a fulcrum, 3b is a pendulum, 4b is a movable electrode, 5b is a fixed electrode, 6b is an insulation seat, 7b and 8b are cables, 9b is a relay circuit, 10b is a relay output.
The platform 1b is positioned horizontally. The movable electrode 4b attached to the pendulum 3b is placed in the center of the fixed electrode 5b in the stationary condition. If there is a vibration, the relative position of the pendulum 3b and the platform 1b changes. The pendulum 3b is supported by the fulcrum 2b so that it may move freely in all directions within a horizontal plane. Therefore, provided the magnitude of the earthquake motion is limited within the amplitude of the inner diameter of the fixed electrode, the movable electrode 4b will come into contact with some part of the interior wall of the fixed electrode 5b.
Since the fixed electrode 5b and the movable electrode 4b are connected with the relay circuit 9b by the cables 7b and 8b, the relay circuit 9b is actuated by the contact of the movable electrode 4b and fixed electrode 5b, and the relay output 10b indicates that the movable electrode 4b contacts with the fixed electrode 5b. The fact that the earthquake motion has reached the set point is detected in this manner.
Although this device is of simple configuration, the size of the fixed electrode 5b determines the set point and no modification of the set point can be made.
FIG. 3(A) and FIG. 3(B) explain the model 3 (detector C). In those FIGS., 1c is a housing, 2c is a pendulum, 3c are contacts, 4c is a set point adjustable screw, 5c is a relay circuit, 6c is a relay output.
The housing 1c is positioned horizontally. The pendulum 2c is standing upright in a stationary condition. The multiple contacts 3c provided on the circumference of the upper part of the pendulum 2c are directly confronting the set point adjustment screws 4c. The distance between these contacts 3c and their respective set point adjustment screws 4c is identical throughout. The set point adjustment screws 4c may be adjusted so that the contact 3c will make a circuit precisely at a point where the value of horizontal motion is to be detected. When the earthquake motion reaches the set point, any one of the multiple contacts 3c makes a circuit, whereby the relay circuit 5c is actuated. Then the relay output 6c becomes inverted from its stationary condition. That the earthquake motion has reached the set point may be detected in this manner.
This device has an advantage in that its set point can be altered by its set point adjustment screw 4c. However, this means that a very careful adjustment of the multiple numbers of the set point adjustment screws 4c is essential. The numbers of the contacts 3c and the set point adjustment screws 4c determine set point errors. Therefore, if errors are to be reduced, the number of the contacts should be increased resulting in increased adjustment work which is troublesome.
In order to overcome the above disadvantages the present applicant proposed the improved vibration detector in U.S. patent application Ser. No. 841,116. Such a vibration detector comprises a stationary housing, a mass pivotally mounted in the housing so that it can deflect in an oscillatory manner from a vertical datum position upon the application of vibration to the housing and returns to the datum position after dessation of the application of the vibration, a light beam sensor having a plurality of cells mounted on the mass or on the stationary housing, a light source mounted on the stationary housing or on the mass, whichever does not have the sensor mounted on it, optical means for focusing a light beam from the source and causing it to be directed on to one or other of the cells in dependence upon whether or not the mass is deflected, and each cell of said beam sensor being arranged to provide an electrical signal according to the cell that the light beam illuminates depending upon the deflection of the mass.
Although the above vibration detector is satisfactory in operation, there are some points which should be improved. For instance, the structure of the mass is complicated as the mass has an optical and/or electrical means. Since the mass must deflect very smoothly, complicated structure of the mass is a necessary disadvantage. Further, as the light path does not amplify the deflection of the mass, the sensitivity of the above vibration detector is rather low.